The invention relates to lifting gear for raising and lowering a load.
Such lifting gear serves for raising and lowering a load, in particular drilling equipment, for example from a platform into a bore hole or onto the seabed. It comprises flexible tension means, of which one end is attached to the drilling equipment. The other end of the tension means is attached to a rotatably mounted winding drum by which the tension means can be wound up and unwound.
At least one rotary driving device is provided for driving the drum. The device usually incorporates a direct current motor. In order to be able to drive the drum over a relatively wide speed range with almost constant high power, there is connected to the direct current motor a change-speed gearbox, of which the input shaft is coupled to the motor and the output shaft to the drum.
In a first embodiment of such known lifting gear the torque which is available is transmitted to the drum by means of a roller-link driving chain. It is true that these so-called chain drives have relatively compact external dimensions, but there is a drawback in that the slack always present in the lower run of the chain drive in so-called xe2x80x9cfour-quadrant operationxe2x80x9d, in which the load can be both accelerated and also braked both on raising of the load and also on lowering it, can switch to the upper run and thereby produce a sudden shock-like operation with substantial peak loads acting on the tension means. A significant danger lies in this, since the tensile strength of the tension means can be exceeded by these peak loads, which can lead to failure of the tension means, with the associated catastrophic consequences.
There is therefore known, from Messrs Wirth Maschinen-und Bohrgerate-Fabrik GmbH of Erkelenz, lifting gear including two drive units each comprising a direct current electric motor with associated change-speed gearbox, in which the gearbox output shafts each carry a respective toothed gear pinion which engages a gearwheel secured on the shaft of the drum. The two rotary driving devices are mounted side by side on an extension of the axis of the drum at one of the end faces of the drum in such a way that the engagements of the two pinions with the gearwheel mounted on the shaft of the drum are offset through 180xc2x0 with respect to the axis of the drum. On the far side, at the other end face of the drum are provided, one following the other looking along the axis of the shaft, a disc brake and an eddy-current brake secured to the shaft of the drum.
Using this gear-driven lifting equipmentxe2x80x94in contrast to the chain-driven lifting gearxe2x80x94it is possible to provide xe2x80x9cfour-quadrant operationxe2x80x9d without this leading to unwanted peak loads in the tension means. Through the possibility of using the driving motors also for braking the rotation of the drum, through the possibility of feeding back directly the electrical energy recovered in the braking process, a substantial saving in energy can be achieved. Furthermore the mechanical disc brake arrangement only comes into use substantially less frequently than in the case of the chain-driven lifting gear, whereby on the one hand the wear to which it is subjected and the down times of the lifting gear resulting from wear, and on the other hand the noise penalties regularly arising with mechanical brakes, are reduced to a minimum.
In fact this gear-driven lifting equipment has proved itself many times in recent years, but the substantial amount of space that it takes up makes it unsuitable in particular for simply replacing the more compact chain-driven lifting gear.
It is known that alternating current rotary driving electric motors have a higher torque over a wider speed range than direct current motors. Accordingly it has become known to equip gear-driven lifting equipment of the kind described above with alternating current electric motors without change-speed gearboxes as a replacement for direct current electric motor/gearbox units, since in this way the structural length of the lifting gear is reduced. However a significant drawback lies in the fact that, in particular when the gear-driven lifting equipment is to be introduced in exchange for a direct current chain-driven lifting gear, the whole electrical installation must be changed over from direct current to alternating current, which generally involves an outlay which is not economically sustainable.
The invention is therefore based on solving the problem of providing lifting gear with which four-quadrant operation is possible but which uses direct current and of which the structural length is reduced.
Pursuant to the invention, at least one rotary drive device is arranged alongside the drum in relation to the axis of rotation of the drum in such a way that the drum and the rotary drive device partially overlap one another in a projection perpendicular to the axis of rotation of the drum. Due to this construction, the structural strength of the lifting gear is significantly reduced. The length of the lifting gear is accordingly determined substantially by the drum and the assemblies which are connected ahead of it and following it in the direction of the axis of the drum.
In a preferred embodiment of the lifting gear according to the invention the driving shaft of the driving motor and the input and output shafts of the gearbox are arranged to lie on a common straight line. Any change of direction of the driving torque which would involve an increased outlay and also power loss is thereby avoided.
The drum is preferably secured to a rotatably mounted drum shaft to rotate with it.
The drum shaft is then preferably connected to the output side of a gearbox unit of which the input side is coupled to the output shaft of the change-speed gearbox.
The gearbox unit is preferably a toothed gear drive which has an idler pinion between output shaft of the change-speed gearbox and the shaft of the drum to bridge the gap between them.
If the drum shaft is coupled at one end to a mechanically acting braking device and at the other end to an electrically acting braking device, then in case of need a high braking torque can be exerted on the drum by operating both braking devices simultaneously without this leading to torsion forces being applied to only one end of the drum shaft, as would be the case if only one end of the drum shaft were coupled to braking devices.
The mechanically acting braking device is preferably a disc brake and the electromagnetically acting braking device is an eddy-current brake.
To increase the torque which can be transmitted to the drum shaft and to achieve emergency running characteristics the lifting gear according to the invention preferably has a second rotary driving device with a rotary driving motor and change-speed gearbox.
In this arrangement the drive shaft of the rotary driving motor and the input and output shafts of the change-speed gearbox of the second rotary driving device are arranged to lie on a common straight line whichxe2x80x94and this is particularly preferredxe2x80x94coincides with the axis of rotation of the output shaft of the change-speed gearbox of the first rotary driving arrangement.
The output shafts are preferably both coupled directly to the input side of the gearbox unit, which can be structurally modified so that they are both operatively connected to the same gearbox input pinion.
If the second rotary driving devicexe2x80x94looking in the direction of the axis of rotation of the electromagnetically acting braking devicexe2x80x94is mounted alongside this braking device, the structural length of this lifting gear equipped with two rotary driving devices is increased either not at all or only insignificantly.
To avoid overloading the driving motors of the rotary driving devices, the change-speed gearboxes are preferably equipped with a safety device which, in the event of a maximum permissible torque at the input shaft of the gearbox being exceeded, automatically change gear to the ratio having the greatest ratio of the speed of the input shaft to the speed of the output shaft.